This invention relates to a drive circuit for driving a piezoelectric transformer which generates an AC voltage using a piezoelectric material and, in particular, to a drive method in an open state of a load or a state where a cold-cathode tube serving as the load does not light up because the cold-cathode tube has a high impedance.
In general, the piezoelectric transformer is an element which comprises a piezoelectric material having first and second primary electrodes and a secondary electrode. The piezoelectric transformer has a resonance frequency. The piezoelectric transformer is resonated by applying an AC voltage having the resonance frequency to the first and the second primary electrodes and the piezoelectric transformer takes a generation voltage due to mechanical vibration out of the secondary electrode. Inasmuch as the piezoelectric transformer is miniaturized and thinned in comparison with an electromagnetic transformer, the piezoelectric transformer is used as a main transformer for use in a back light electric source for a liquid crystal display.
A conventional drive circuit for the piezoelectric transformer is proposed in Japanese Patent Application No. Hei 7-69,207 which is published as Japanese Unexamined Patent Publication of Tokkai No. Hei 8-275,553 or JP-A 8-275,553 of Oct. 18, 1996 after making an application for the instant Japanese Patent Application No. Hei 8-203,579 of Aug. 1, 1996.
According to JP-A 8-275,553, the drive circuit comprises an inverter circuit and a frequency control circuit. The frequency control circuit is supplied with, as a feedback current, a load current from a load. The load may be a cold-cathode tube. Response to the feedback current, the frequency control circuit supplies the inverter circuit with a frequency control signal indicative of a drive frequency for the piezoelectric transformer. The inverter circuit is supplied with a DC power source voltage as an input DC voltage directly. Response to the frequency control signal, the inverter circuit converts the input DC voltage into a main drive voltage. The main drive voltage is supplied to the first and the second primary electrodes of the piezoelectric transformer as an input AC voltage. Responsive to the input AC voltage, the piezoelectric transformer carries out resonance operation to produce an output AC voltage from a secondary electrode thereof. The output AC voltage is supplied to the load.
The frequency control circuit has a normal frequency sweeping range which is defined between a normal upper limit frequency and a normal lower limit frequency. The normal frequency sweeping range includes the resonance frequency of the piezoelectric transformer. The frequency control circuit sweeps the drive frequency within the normal frequency sweep range. Responsive to a reset signal, the frequency control circuit is reset to shift the drive frequency from the normal lower limit frequency to the normal upper limit frequency.
Another conventional drive circuit for the piezoelectric transformer is proposed in Japanese Patent Application No. Hei 6-241,049 which is published as Japanese Unexamined Patent Publication of Tokkai No. Hei 8-107,678 or JP-A 8-107,678 of Apr. 23, 1996 prior to making the application for the instant Japanese Patent Application No. Hei 8-203,579 of Aug. 1, 1996.
According to JP-A 8-107,678, the drive circuit prevents destruction of the piezoelectric transformer when the output AC voltage of the piezoelectric transformer is to high. The drive circuit comprises a driving circuit, a load-current comparing circuit, an output-voltage comparing circuit, and a frequency sweeping oscillator. The load-current comparing circuit converts load current into a DC voltage and then compares the DC voltage with a load-current reference voltage which corresponds to a desired load-current value. The load-current comparing circuit produces a load-current comparison result signal which is supplied to the frequency sweeping oscillator. On the basis of the load-current comparison result signal, the frequency sweeping oscillator determines a direction in a frequency sweeping of the drive frequency. The frequency sweeping oscillator has upper and lower limit frequencies for the frequency sweeping. The output-voltage comparing circuit divides and rectifies the output AC voltage from the piezoelectric transformer to produce a divided-rectified voltage and then compares the divided-rectified voltage with an output-voltage reference voltage which corresponds to a desired output AC voltage. The output-voltage comparing circuit produces an output-voltage comparison result signal which is supplied to the frequency sweeping oscillator. On the basis of the output-voltage comparison result signal, the frequency sweeping oscillator determines the direction in the frequency sweeping of the drive frequency. The driving circuit is connected to the first and the second primary electrodes of the piezoelectric transformer. Responsive to a frequency control signal indicative of the drive frequency, the driving circuit drives the piezoelectric transformer.
Still another conventional drive circuit for the piezoelectric transformer is proposed in Japanese Patent Application No. Hei 7-264,081 which is published as Japanese Unexamined Patent Publication of Tokkai No. Hei 9-107,684 or JP-A 9-107,684 of Apr. 22, 1997 after making the application for the instant Japanese Patent Application No. Hei 8-203,579 of Aug. 1, 1996.
According to JP-A 9-107,684, the drive circuit is operable at a high efficiency in a wide input DC voltage range. The drive circuit comprises not only the inverter circuit and the frequency control circuit but also a drive voltage control circuit and a dimmer circuit. The DC power source voltage is supplied to the inverter circuit via the drive voltage control circuit. In other words, the drive voltage control circuit supplies the inverter circuit with an input DC voltage having controllable input power. The inverter circuit includes first and second subsidiary transformers supplied with first and second subsidiary drive voltages, respectively. The first subsidiary drive voltage is supplied to the drive voltage control circuit. The frequency control circuit includes a voltage controlled oscillator which oscillates a chopping wave signal as well as the frequency control signal. The chopping wave signal is supplied to the drive voltage control circuit. Responsive to the chopping wave signal and the first subsidiary drive voltage, the drive voltage control circuit controls the input power of the input DC voltage. Responsive to a dimmer voltage, the dimmer circuit produces a dimmer control signal which is supplied to the drive voltage control circuit.
However, the above-mentioned conventional drive circuits have two problems as follows. A first problem is that electronic parts composing the inverter circuit develop heat in a case where the piezoelectric transformer is put into an abnormal state or has high impedance. A second problem is that it is in design difficult to make the normal upper limit frequency of the voltage controlled oscillator shift downward. This is because the drive frequency must be laid in a frequency sweeping range of the frequency control circuit under all conditions. To resolve the first problem must shift the normal upper limit frequency to a lower frequency. On the contrary, to resolve the second problem must shift the normal upper limit frequency to a higher frequency. Accordingly, the first and the second problems conflict with each other and it result in difficult to design the drive circuit for the piezoelectric transformer. That is, the above-mentioned conventional drive circuits do not take into account in the case where the piezoelectric transformer is put into the abnormal state.